Thermoplastic polyesters are step growth polymers that are useful when made to high molecular weights. The first step in a common method of producing a polyester such as polyethylene terephthalate (PET) is an esterification or ester-exchange stage where a diacid (typically terephthalic acid) reacts with an appropriate diol (typically ethylene glycol) to give a bis(hydroxyalkyl)ester and some linear oligomers. Water is evolved at this stage and is usually removed by fractional distillation.
Pursuant to the goal of making polyethylene terephthalate and other polyesters, a great deal of patent literature is dedicated to the describing processes for preparing terephthalic acid/ethylene glycol mixtures suitable as starting material. In general, these inventions describe specific mixing schemes with a purified terephthalic acid solid and liquid ethylene glycol as starting materials. Additionally, there is substantial body of literature devoted to producing a purified terephthalic acid in the powder form that is suitable for use in producing PET. The objective of this invention is to describe a process by which a terephthalic acid/ethylene glycol mixture suitable as starting material for polyester production is obtained from a decolorized terephthalic acid solution without isolation of a substantially dry terephthalic acid solid.
A number of processes for producing the purified terephthalic acid solid have been developed and are commercially available. Usually, the purified terephthalic acid solid is produced in a multi-step process wherein a crude terephthalic acid is produced. The crude terephthalic acid does not have sufficient quality for direct use as starting material in commercial PET. Instead, the crude terephthalic acid is usually refined to purified terephthalic acid solid.
Liquid phase oxidation of p-xylene produces crude terephthalic acid. The crude terephthalic acid is dissolved in water and hydrogenated for the purpose of converting 4-carboxybenzaldehyde to p-toluic acid, which is a more water-soluble derivative, and for the purpose of converting characteristically yellow compounds to colorless derivatives. Any 4-carboxybenzaldehyde and p-toluic acid in the final purified terephthalic acid product is particularly detrimental to polymerization processes as they act as a chain terminator during the condensation reaction between terephthalic acid and ethylene glycol in the production of PET. Typical purified terephthalic acid contains on a weight basis less than 25 parts per million (ppm) 4-carboxybenzaldehyde and less than 150 ppm p-toluic acid.
The crude terephthalic acid typically contains on a weight basis from about 800 to 7,000 parts per million (ppm) 4-carboxybenzaldehyde and about 200 to 1,500 ppm p-toluic acid as the main impurities. The crude terephthalic acid also contains lesser amounts, about 20-200 ppm range, of yellow color aromatic compounds having the structures of benzil, fluorenone, and/or anthraquinone, which are characteristically yellow compounds as impurities resulting from coupling side reactions occurring during oxidation of p-xylene. It is necessary to purify the crude terephthalic acid when using it as a starting material for producing polyester fiber, which requires a purified terephthalic acid as a starting material.
Such a purification process typically comprises adding water to the crude terephthalic acid to form a crude terephthalic acid solution, which is heated to dissolve the crude terephthalic acid. The crude terephthalic acid solution is then passed to a reactor zone in which the solution is contacted with hydrogen in the presence of a heterogeneous catalyst at temperatures of about 200° to about 375° C. This reduction step converts the various color bodies present in the crude terephthalic acid to colorless products. The principal impurity, 4-carboxybenzaldehyde, is converted to p-toluic acid.
Typical crude terephthalic acid contains excessive amounts of both 4-carboxybenzaldehyde and p-toluic acid on a weight basis. Therefore, to achieve less than 25 ppmw 4-carboxybenzaldehyde and less than 150 ppmw p-toluic acid in the purified terephthalic acid requires mechanisms for purifying the crude terephthalic acid and removing the contaminants.
Subsequent separation and isolation of the purified terephthalic acid can be accomplished via a wide variety of separation methods including crystallization, centrifugation, filtration, extraction and combinations thereof followed by drying. These processes are described in U.S. Pat. Nos. 4,500,732; 5,175,355; and 5,583,254; all of which are herein incorporated by reference. It is necessary to perform a separation step due the nature of the crude terephthalic acid feedstock to the hydrogenation process.
A number of processes have been developed for producing a purified terephthalic acid solid from crude terephthalic acid. In general, the common features among these processes are as follows:
Step (1) is decolorization of the crude terephthalic acid usually via hydrogenation treatment in an aqueous medium;
Step (2) is purification/separation of the terephthalic acid from partial oxidation products usually via fractional crystallization followed by liquor exchange with contaminant-free water; and
Step (3) is production of a solid purified terephthalic acid product with consistent material handling properties usually via crystallization of terephthalic acid followed by drying of purified terephthalic acid from water.
The resultant purified terephthalic acid powder along with ethylene glycol are starting materials in the production of polyesters specifically PET. Because the difficulty in handling, mixing, and dissolving terephthalic acid solids, the purified terephthalic acid solid is usually mixed with ethylene glycol to form a paste prior to introduction into an esterification reactor system.
In the present invention, a novel process has been discovered resulting in fewer steps than the currently employed processes. The primary utility of the invention is reduction of capital and operating costs associated with the isolation of a terephthalic acid powder. In the conventional approach toward producing terephthalic acid, the post-hydrogenated aqueous solution is passed to a series of crystallizer vessels for the purpose of purifying the terephthalic acid by crystallization and for the purpose of obtaining a uniform particle size distribution necessary for good flowability of purified terephthalic powder. Further, the p-toluic acid contaminated mother liquor from the crystallization process must be removed prior to a drying step to isolate the purified terephthalic powder.
In on embodiment of the present invention, the crude terephthalic acid solution with low concentrations of p-toluic acid and 4-carboxybenzaldehyde is hydrogenated to form a decolorized terephthalic acid solution. Starting with crude terephthalic acid with low concentrations of the p-toluic acid and 4-carboxybenzaldehyde eliminates the need for separation of p-toluic acid-contaminated mother liquor from the terephthalic acid. Hence, the decolorized terephthalic acid solution can be directly combined with ethylene glycol in an esterification zone to produce a terephthalic acid/ethylene glycol mixture. By bypassing conventional processes for producing a purified terephthalic acid powder, the need for the equipment necessary to purify and isolate purified terephthalic powder is eliminated.
Another surprising and seemingly contradictory aspect of the invention is the benefits of addition of large amounts of water to the esterification reaction starting materials. This is directly contrary to accepted esterification procedures. The esterification reaction:RCOOH+R′OH→RCOOR′+H2Ois generally not complete. The water formed in the course of the reaction tends to react with the ester to hydrolyze it, i.e. to regenerate the original alcohol and acid. In order to drive the reaction toward the ester, the prior art teaches removal of water from the system by a variety of methods such as distillation or dehydration with a hydrophilic compound. According to conventional esterification procedures, it is non-intuitive to add large amount of the water to the acid/alcohol starting material.